A bifurcation phenomenon of Stokes curves around a double turning point, and influence of virtual turning points upon the transition probabilities for three-level systemsA bifurcation phenomenon of Stokes curves around a double turning point, and influence of virtual turning points upon the transition probabilities for three-level systems二重変わり点における Stokes 曲線の分岐現象, そして三準位系の遷移確率に対する仮想変わり点の影響